The top symbol shows a simple, direct operated pressure relief valve.
Note how the arrow is shown in it's deactivated position e.g. with the spring force higher than the pressure in the pilot. The pilot pressure also comes straight from the supply line upstream of the valve showing that as the pressure before the valve increases it pushes the arrow against the spring and relieves the pressure.
The lower symbol shows a pilot operated pressure relief valve. This valve operates in the same way as the top, direct operated valve except that it will have at least two stages of valve within it. Learn more about relief valves here.
These symbols show similar valves to the ones above. Both work in the same way and will relieve the pressure when it gets too high but these also have an electrical load or unload facility. Neither shows enough detail to say whether the solenoid needs to be energised to make the valve raise pressure or de-energised to raise pressure.
The bottom symbol shows two stage pilot operation with an external Y drain. This may be important for holding a constant or stable pressure because without it the balanced spring chamber pressure would be fed from the valve tank return line which could be subject to pressure peaks or oscillations.
Pressure reducing valve symbols look very similar to those for pressure relief valves. This is because the valves work in similar ways except that the pressure sense line feeds from downstream of the reducing valve instead of upstream as in the relief valve. This means that the reducing valve controls the downstream pressure rather than the upstream pressure. As such the reducing valve cannot generate a downstream pressure that is not there but it can limit its value. Not how the arrow is shown in the middle positions indicating that it is normally open but closes the flow off when the pressure feed forces the arrow against the adjustable spring.
The top valve shows a direct operated valve with external Y drain line but only 2 ports.
The middle symbol has a third tank line port and arrowheads pointing in both directions. In this valve, if the pressure downstream rises above the reducing valve's setting then the valve will vent this pressure down the third line to provide a constant pressure in the sensed port.
The lower symbol shows a pilot operated version of the middle, direct operated valve.
The symbol on the left shows a sequence valve with a manually adjustable set point and the one on the right has an externally controlled pilot switching pressure and internal drain.
These symbols show 2-way sequence valves with an external pilot and internal drain on the left and external pilot and drain on the right.
The third symbol image shows 3-way sequence valves with internal pilot and drain on the left and external pilot and drain on the right.
The final symbol shows a kick down valve which is commonly used to as a pump unloading valve, as they have a lower standby pressures e.g. energy loss, when open.
All pressure control valves operate in proportion to their pilot pressures, so the symbol does not include the extra lines around the box that designates the proportional version of a directional valve. The key difference in the symbol between the standard on/off electrical control and a proportional electrical control is the arrow through the solenoid and possibly the dotted triangle symbol to show internal electronics.
Proportional pressure reducing valves are very common on mobile control block as they are used to apply pressure on the ends of the spools that control the proportional directional valves.